Three little stars beside Jupiter

Galileo noted three faint stars in a line beside Jupiter and thought nothing much of it. The following night they had moved, and the night after that one of them had gone.

It took him a week of watching to be sure they were going round the planet rather than drifting past it, and a fourth turned up on the thirteenth. Something orbiting something other than the Earth was the part that mattered, and he knew it.